Method for producing organically developable, photopolymerizable flexographic printing elements on flexible metallic supports

ABSTRACT

The invention relates to a method for producing organically developable, photopolymerizable flexographic printing elements on flexible metallic supports by: coating a flexible metallic support with a tack-free adhesive lacquer, which is resistant to swelling and which is insoluble in printing inks and in organic developers; applying an elastomeric photopolymerizable layer to a covering film, and; laminating the photopolymerizable layer onto the metallic support coated with the adhesive lacquer. The invention also relates to a photopolymerizable flexographic printing element comprised of a photopolymerizable layer that is applied to a flexible metallic support by means of a tack-free adhesive layer, which is resistant to swelling and which is insoluble in printing inks and in organic developers.

[0001] The invention relates to a process for the manufacture oforganically developable, photopolymerizable flexographic elements onflexible metallic supports. The invention also relates to organicallydevelopable, photopolymerizable flexographic elements on flexiblemetallic supports in which the photopolymerizable layer is attached tothe metallic support by means of a tack-free adhesive film that isnon-swelling and insoluble in printing inks and organic developers. Theinvention also relates to an organically developable, photopolymerizableflexographic element comprising a photopolymerizable layer which isattached to a flexible metallic support by means of an adhesive filmwhich is non-swelling and insoluble in printing inks and organicdevelopers.

[0002] Photopolymerizable flexographic elements for the manufacture offlexographic printing plates usually consist of a flexible substrate, anadhesive layer, a photopolymerizable layer, a release layer and also aprotective film for the photopolymerizable layer, which is removed priorto exposure to light. The supports used in commercial flexographicelements are usually PET films.

[0003] Perfectly registered set-up of PET-supported flexographicprinting plates on the impression cylinder is usually effected usingdouble-sided adhesive tape. Usually the flexographic printing plates aremounted outside the printing press on the dismantled impressioncylinder, for which purpose frequently special mounting equipment isused. The impression cylinder thus provided with the plates is thenremounted in the printing press. If it is necessary to correct theposition of the flexographic plate, the impression cylinder to which itis stuck must be again dismantled from the printing press and theflexographic plate unstuck. Then a new piece of double-sided adhesivetape is stuck to the impression cylinder, onto which the flexographicplate is restuck, and the impression cylinder is again remounted in thepress. This, however, is a complicated and time-consuming procedure.

[0004] In the prior art, water-developable, photopolymerizablehigh-pressure plates are laminated to PET supports or metallic supportsof aluminum or steel. To ensure good adhesion of the photopolymerizablelayer on metallic supports, used can be made, for example, of acombination of two adhesive layers comprising a prime coating and anintermediate layer. For examples of this usage reference is made to DE-A3,045,516 or EP-A 333,012. Printing plates on thin metallic supports aremuch easier to mount on impression cylinders. Attainment of perfectregistration is further alleviated by providing high-pressure plateswith a thin support of magnetizable steel, in known manner, (Print-itNo. 2, June 1999, BASF Drucksysteme GmbH, page 12-13). Suchhigh-pressure plates can be simply and quickly mounted on magneticimpression cylinders, in perfect registration.

[0005] Flexographic printing plates on metallic supports are well known.Patents or patent applications relating to flexographic printing platesfrequently mention, in the description, that metals, as a general class,are suitable for use as materials for the supports. Reference is madehere, by way of example, to EP-A 992,849 (section [0030]) and EP-A474,178 (page 3, lines 50-54).

[0006] However, the working examples of such patents or patentapplications usually illustrate the use of flexographic elementssupported by only polymer films, particularly PET films. Only EP-A332,983 gives working examples of flexographic printing plates onmetallic supports. However, the flexographic element disclosed in saidreference is not an organically developable element based onthermoplastic elastomeric block copolymers but a water-developableelement based on specific polyvinylalcohol derivatives, as bindingagents. Such binding agents are taken from high-pressure platetechnology, and such flexographic printing plates can only be used withUV links. No organically developable flexographic printing plates onflexible metallic supports are as yet commercially available.

[0007] Hitherto only photopolymerizable flexographic elements on adouble support of PET film and an aluminum sheet (eg Nylocoat® LA 116(BASF), Cyrel® CLAM (Du Pont)) have been commercially available. Theseplates comprise a photopolymeric layer which is not directly attached tothe aluminum support, but they are conventional flexographic printingplates supported on a PET film, which is stuck to an additional aluminumsupport by means of, say, double-sided adhesive tape. This requires anadditional process step, which is very time-consuming andlabor-intensive. There is also the risk that when the plate is beingdeveloped the washout agent may cause the adhesive tape to lift from thealuminum support. Moreover, the interlayer adhesion between the PET filmand the aluminum sheet can break during the printing process due tomechanical stresses. Since the loss of adhesion can alter the register,it is an exceptionally undesirable effect.

[0008] The flexographic printing plates on double PET/aluminum supportsdescribed therein are mainly used for upgrading products of sheet-fedoffset machines, for example by lacquering or gold printing (cf eg“Inlineveredelung ueber Flexolackierwerke”, published by DeutscherDrucker 29 (1999) w2- w6). Flexographic printing plates used for thispurpose are therefore also referred to as coating plates. In this fieldparticular importance is attached to accuracy of register. Modernflexocoating machines in sheet-fed offset machines are frequentlyequipped with quick-action clamping bars or with fully automatic platedraw-in devices only suitable for drawing in printing plates having analuminum support. Customers in offset printshops prefer flexographicprinting plates on double PET/aluminum supports by reason of theirdistinctly higher dimensional stability as opposed to flexographicprinting plates on film supports. In production printing, aluminum isdistinctly more resilient to mechanical stresses than polyester. Even inthe case of repeat orders involving repeated clamping and unclamping,the dimensional stability of the printing plates on aluminum supports isassured.

[0009] In view of the many advantages of flexographic printing plates onmetallic supports, it would be desirable to apply the elastomericphotopolymerizable layer directly to the metallic support, in order toavoid the aforementioned complicated and elaborate manufacturingprocess.

[0010] In practice, however, the skilled person wishing to use metallicsupports for flexographic printing plates faces a number of problems.

[0011] Photopolymerizable flexographic elements are usually prepared bymelt extrusion. The photopolymeric composition is discharged through aslot die in between a protective film and a supporting PET film and theresulting composite is calendered. However, this technique cannot besimply transferred to supporting foils. Apart from the fact that it isindustrially very exacting to carry out calendering on a metallicsupport, there is primarily the problem of corrugation. Due to theirdifferent coefficients of thermal expansion the metallic support and theprotective film show different degrees of shrinkage when themelt-extruded photopolymeric composition is cooled. The difference inthe shrinking behavior of the metallic support and the protective filmleads to warping of the flexographic element, which is manifested bycorrugation. Such flexographic elements cannot be used by reason oftheir high thickness tolerances.

[0012] The first step in the manufacture of flexographic printing platesof photopolymerizable flexographic elements is usually a so-called backexposure through the transparent supporting PET film. This serves interalia to establish the relief depth and also to ensure that theindividual halftone dots are firmly fixed to the substrate. The backexposure first of all causes a portion of the photopolymerizable layerto be completely polymerized, namely the lower region in contact withthe supporting film. The upper portion of the layer remainsunpolymerized at this stage. The upper portion is then converted to theactual printing relief in a second process step by front imaging. Thusthe individual halftone dots of the flexographic printing plate do notadhere to the supporting film itself but are located on a layer ofpolymerized material in full-surface adhesive contact with thesupporting film. The back exposure method is illustrated for example in“Technik des Flexodrucks”, pp 148 et seq, 4th Edition, 1999, CoatingVerlag, St. Gallen, Switzerland. It is of course impossible to carry outany exposure through the back of a flexographic printing plate on ametallic support. Consequently, when the flexographic element isdeveloped following front exposure through the negative theunpolymerized material of the unexposed regions is removed right down tothe support. Thus there is the risk that particularly those individualhalftone dots which have only a small base area and weak adhesion willbe torn away from the support during development or during the printingoperation.

[0013] It is thus an object of the invention to provide a simple andcost-effective process for the manufacture of organically developableflexographic printing plates on metallic supports.

[0014] Accordingly, we have found a process for the manufacture oforganically developable, photopolymerizable flexographic elements onflexible metallic supports by coating a flexible metallic support with atack-free adhesive layer which is non-swelling and insoluble in printinginks and organic developers, applying an elastomeric, photopolymerizablelayer to a protective film, and laminating the photopolymerizable layerto the metallic support coated with said adhesive layer.

[0015] In another embodiment of the invention we have found a processfor the manufacture of organically developable, photopolymerizableflexographic elements on flexible metallic supports for makingflexographic printing plates by coating a flexible, metallic supportwith a tack-free adhesive layer which is non-swelling and insoluble inprinting inks and organic developers, melt extruding an organicallydevelopable, elastomeric photopolymerizable composition and dischargingit through a slot die in between a protective film or protective elementand a temporary supporting film, and then calendering the composite,peeling off the temporary supporting film, laminating thephotopolymerizable layer on its side remote from the protective film tothe flexible metallic support coated with the adhesive layer.

[0016] We have also found an organically developable, photopolymerizableflexographic element composed of a photopolymerizable layer which isapplied to a flexible metallic support by means of a tack-free adhesivelayer which is non-swelling and insoluble in printing inks and organicdevelopers.

[0017] The following statements serve to describe the process of theinvention in more detail.

[0018] The metallic supports used for the flexographic elements areflexible. For the purposes of the invention, “flexible” should be takento mean that the supports are thin enough to be bent around impressioncylinders. On the other hand they are dimensionally stable and thickenough not to be buckled during production of the flexographic elementor when the finished printing plate is mounted on the impressioncylinder.

[0019] Suitable flexible metallic supports are primarily thin sheets ormetal foils of steel, preferably stainless steel, magnetizable springsteel, aluminum, zinc, magnesium, nickel, chromium or copper, thesemetals being alloyed if desired. Alternatively, use can be made ofcombined metallic supports such as steel plates coated with tin, zinc,chromium, aluminum, nickel or combinations of various metals, oralternatively metal supports obtained by laminating metal sheets of thesame or different types. Furthermore, pretreated metal sheets, such ascoslettized or chromatized steel sheeting or anodized aluminum sheets,may alternatively be used. The sheets or foils are usually degreasedprior to use. Preferably use is made of supports of steel or aluminumand more preferably of magnetizable spring steel.

[0020] The thickness of such flexible metallic supports is usuallybetween 0.025 mm and 0.4 mm, and is governed by the desired degree offlexibility and also the type of metal used. Supports of steel usuallyhave a thickness ranging from 0.025 to 0.25 mm and preferably from 0.14to 0.24 mm. Supports of aluminum usually have a thickness ranging from0.25 to 0.4 mm.

[0021] The flexible metallic support is provided with a tack-freeadhesive layer which is non-swelling and insoluble in printing inks andorganic developers. The adhesive layer serves to provide good adhesionbetween the flexible metallic support and the photopolymerizable layerto be later applied thereto, so that the printing elements obtained byimaging the photopolymerizable layer are not torn off or broken off, ordo not detach themselves, from the substrate when the plate is beingdeveloped or during printing.

[0022] The surface of the adhesive layer is tack-free. The degree ofnontackiness can be assessed qualitatively by the person skilled in theart by feeling the layer, or determined quantitatively for example bytaking pendulum tack readings. Furthermore, the adhesive layer isinsoluble and non-swelling in conventional organic developers used forflexographic elements and also in conventional organic solvents ororganic component-containing solvents in printing inks. Typical organicdeveloping agents for flexographic elements comprise for examplecarboxylates, hydrogenated petroleum fractions or terpenes in admixturewith alcohols. Reference is made for example to the publications EP-A332,070, EP-A 463,486, EP-A 28,676, EP-A 433,374 or WO 93/10484. Typicalsolvents for flexographic inks comprise for example alcohols, such asethanol or isopropanol, optionally in admixture with water.

[0023] Theoretically, any type of adhesive layer can be used inexecution of the present process, provided that the aforementionedminimum requirements are satisfied.

[0024] An example of a suitable adhesive layer for execution of theprocess of the invention has been found to be one which comprises abinding agent which is embedded in a suitable polymer matrix. Discretedomains of elastomeric binding agent and matrix can usually be seenunder the microscope.

[0025] Examples of suitable binding agents for use in the adhesive layercomprise elastomeric or elastothermoplastic polymers, such as are alsousually employed for the manufacture of relief printing plates, such aspolymers or copolymers of 1,3-dienes or SIS or SBS blockcopolymers.Alternatively, mixtures of two or more different elastomeric bindingagents may are used.

[0026] The amount of elastomeric binding agent in the adhesive layer isdetermined by the person skilled in the art in accordance with theproperties desired. It is usually from 10 to 70 wt %, based on the totalcomponents of the adhesive layer, particularly from 10 to 45 wt % andmore particularly from 15 to 35 wt %.

[0027] The polymer matrix is usually a crosslinked polymer matrixobtained by means of a suitable crosslinking system. The crosslinkedpolymer matrix can be obtained thermally by polycondensation orpolyaddition of suitable monomers or oligomers, for example by thereaction of polyurethanes and suitable hydroxyl group-containingcompounds such as hydroxyl group-containing polyurethane resins orpolyester resins to form crosslinked polyurethanes.

[0028] Another suitable adhesive layer has been found to be one based onmodified polyvinyl alcohols containing radically crosslinkableside-groups. This embodiment of the adhesive layer comprises a partiallysaponified polyvinyl alcohol which has been esterified by methacrylicanhydride. The proportion of radically crosslinkable methacrylate groupsis from 1 to 10 mol % and preferably from 3 to 5 mol %. The adhesivelayer further comprises bis(N-methylo-lacrylamide)ethylene glycol etherand a dialdehyde.

[0029] The adhesive layers may also contain further components andauxiliaries such as additional binding agents for influencing theproperties, dyes, pigments or plasticizers.

[0030] When executing the process of the invention, it has been found tobe particularly advantageous to add a migration-stable ultravioletabsorber to the adhesive layer. In this way backscattering of UV lightthrough the metallic support during imaging of the flexographic elementis prevented or at least greatly reduced. Such back-scattered light canlead to unwanted polymerization in areas of the photopolymerizable layerwhere absolutely no polymerization should take place.

[0031] The addition of migration-stable ultraviolet absorbers to theadhesive layer advantageously increases the exposure latitude of theflexographic elements. Furthermore the bases of the individual halftonedots become more slender, so that the dot gain of the finishedflexographic printing plate during printing is less than when no suchultraviolet absorber is added. The migration-stable ultravioletabsorbers used can be primarily ultraviolet-absorbing substances such asexhibit a high molecular weight and show only a low tendency to migrate.Particularly advantageous are ultraviolet absorbers having reactivegroups, so that they can be chemically embedded in the polymer matrix.An example thereof is a hydroxyl group-substituted Michler's ketone,which can be embedded in a polyurethane matrix.

[0032] Manufacture of the adhesive layer is usually carried out bydissolving the components of the adhesive layer in suitable solvents,mixing them vigorously, filtering the solution if necessary and applyingthe mixture to the flexible metallic supports. Application is preferablycarried out using a flow coater, the thickness of the coating beingcontrolled by varying the gap-width of the coating nozzle. Followingapplication, the solvent is evaporated off. In the case of thermalsystems, crosslinking is achieved by heating for a short period. In thecase of photochemical systems, crosslinking is effected afterevaporation of the solvent by UV irradiation. The residual solventcontent in the layer should be below 5 wt % based on all components ofthe layer.

[0033] The thickness of the adhesive layer is usually from 5 to 100 μm,preferably from 10 to 50 μm and more preferably from 15 to 30 μm.

[0034] Alternatively, use can be made of a number of superimposedadhesive layers of the same, approximately the same, or different,compositions.

[0035] Another process step comprises the production of an elastomeric,organically developable, photopolymerizable layer for the flexographicelement comprising at least one elastothermoplastic binding agent, oneor more ethylenically unsaturated monomers, at least one photoinitiatoror photoinitiator system and, optionally, other components such asplasticizers, inhibitors for thermally initiated polymerization, dyes,pigments, phototropic additives, antioxidants, other binding agents orextrusion aids. Particularly suitable elastothermoplastic binding agentsare the conventional SIS- or SBS-type triblock copolymers. Suchcompositions for the manufacture of organically developable,photopolymerizable flexographic elements are well known to the personskilled in the art and are disclosed for example in DE-A 2,215,090, EP-A084,851, EP-A 819,984 or EP-A 553,662. From the possible components andtheir proportions the person skilled in the art will make a suitableselection governed by the desired properties of the flexographic elementor the flexographic printing plate.

[0036] The photopolymerizable layer can be made, for example, by pouringor melt extrusion. The thickness of the photopolymerizable layer can bevaried by the person skilled in the art according to the desiredproperties of the printing forme. A thickness between 0.5 mm and 1.5 mmhas proven successful for flexographic elements on flexible metallicsupports.

[0037] Manufacture by pouring involves the dissolution of all componentsof the photopolymerizable layer in a suitable solvent to give a clearsolution, which is then poured onto a protective film or protectiveelement followed by evaporation of the solvent. The said film serves asa substrate for the photopolymerizable layer during the pouringoperation and later functions as a protective film for the finishedflexographic element. The protective film prevents thephotopolymerizable flexographic element from being damaged duringstorage or transfer and must be peeled off prior to imaging of theelement. The residual solvent content of the layer should be as low aspossible, in order to avoid the formation of bubbles or blisters due toslow evaporating of residual solvent. Usually the residual solventcontent of the photopolymerizable layer should be less than 5 wt % basedon the sum of all components of the layer. The residual solvent contentis preferably less than 3 wt %.

[0038] The protective film used is usually a PET film, which may bemodified, for example siliconized. Alternatively, protective elementsmay be used which comprise further layers in addition to the protectivefilm.

[0039] An example of another such layer is a release layer locatedbetween the photopolymerizable layer and the protective film and solublein the developing agent for the photopolymerizable layer. It frequentlyconsists of suitable polyamides and facilitates stripping of theprotective film prior to using the flexographic element and alsofacilitates positioning of the photographic negative for imaging.

[0040] Another example is a comparatively thin, photopolymerizable, toplayer located over the photopolymerizable layer. Such a top layer isdisclosed for example in EP-A 084,851. This two-layer arrangement hasthe advantage that the properties of the surface of the printing forme,such as ink transfer, can be changed without influencing theflexotypical properties of the printing forme, such as hardness orelasticity. Thus surface properties and layer properties can be modifiedindependently in order to acquire optimum printing results.

[0041] An IR-ablative layer is another example. This can be on thephotopolymerizable layer or alternatively on the top layer, if present.Such layers usually comprise at least one binding agent and carbonblack. They serve to enable direct digital imaging of thephotopolymerizable flexographic element by means of an IR laser.Examples of such layers are given in EP-A 767,407 or EP-A 654,150.

[0042] The cast elastomeric photopolymerizable layer is placed with itsside remote from the protective film or element against the adhesivecoating on the flexible metallic support and is laminated to saidsupport. A particularly suitable laminating solvent for thephotopolymerizable layer containing elastothermoplastic binding agentsis toluene. However, lamination can be effected without any solvent, forexample by heating (thermal lamination). To this end the elastomeric,photopolymerizable layer is heated to a temperature ranging from 50° to1000°C., preferably from 60° to 80°C., shortly before entering thecalender nip either by means of a stream of hot-air or alternativelywith the aid of an infra-red radiator (eg Heraeus). Both solventlamination and thermal lamination serve to increase the degree ofadhesion to the metallic support. In another embodiment of the processof the invention the photopolymerizable layer is made in known manner bymelt extruding the composition described above, discharging it through aslot die in between a protective film or element and a temporarysupporting film, after which the composite formed is calendered.

[0043] The temporary supporting film used may be, say, a PET film, whichmay be modified, eg siliconized, to facilitate subsequent removalthereof by peeling.

[0044] Further processing involves peeling the temporary supporting filmfrom the photopolymerizable layer and then laminating the latter in theabove manner to the flexible metallic support coated with the adhesivelayer.

[0045] Manufacture of the photosensitive flexographic elements may alsobe effected by pouring a photosensitive layer directly onto the supportcoated with the adhesive layer, and then laminating the protective filmor element thereto.

[0046] The photopolymerizable flexographic elements produced by theprocess of the invention on a metallic support exhibit excellentadhesion of the photopolymerizable layer to the metallic support. Theycan be used for making flexographic formes in known manner. The onlydifference is that the back exposure through the support, asconventionally carried out on flexographic elements on PET supports, isomitted. Making a flexographic forme can on the one hand involve, inclassical manner, peeling the protective film, positioning aphotographic negative, imaging with actinic light, developing, dryingand, optionally, applying aftertreatment. Alternatively, it can be madeusing a digital imaging technique. To this end, a photopolymerizableflexographic element is used which has an IR-ablative layer. Making theflexographic forme then involves the steps of peeling off the protectivefilm, imagesetting the IR-ablative layer, imaging with actinic light,developing, drying and, optionally, applying aftertreatment. Furtherdetails on both processes are known to the person skilled in the art andare disclosed for example in EP-A 992,849.

[0047] The adhesive layer used in the process of the invention causes,also under the influence of the organic developing agents in which theadhesive layer is neither capable of swelling nor soluble, an excellentdegree of adhesion, so that even fine halftone dots are firmly fixed tothe support, although the relief layer is completely removed in theunexposed regions, where the support carrying the adhesive layer becomesuncovered. Very little dust accumulates in the uncovered regions duringprinting. Interruptions of the printing process for removal of dust fromthe printing plate occur less frequently.

[0048] The fact that the metallic support coated in the manner hereinproposed is free from tack contributes further to particularlyeconomical production, for the support can be stacked or rolled upduring production without sticking together and without any need forfurther measures, such as the insertion of paper to form an intermediatelayer.

[0049] The following examples serve to illustrate the invention ingreater detail without restricting the scope of the invention thereto.

EXAMPLE 1

[0050] (a) Making the Adhesive Layer

[0051] There was prepared, with vigorous mixing, a mixture of an SBSblock copolymer (Kraton D 4150, 19.5 wt %), an oligomeric polyurethaneresin containing OH groups (NPP 262, 35 wt %), a copolyester (Vitel3300, 35 wt %), an aromatic isocyanate (Desmodur L 67, 9.5 wt %) and anultraviolet absorber (hydroxy Michler's ketone, 1 wt %) in THF, and thismixture was poured onto an aluminum support (thickness 0.3 mm), afterwhich the solvents were evaporated and the residue crosslinked byheating to 170° C. for 2.5 min.

[0052] There was obtained an adhesive layer having a thickness of from12 to 15 μm. Examination under the microscope revealed rubber particlesof from 80 to 100 μm in diameter in a continuous matrix. The adhesivelayer on the support was tack-free. The coated aluminum sheets could bestacked without sticking together.

[0053] (b) Making the Photosensitive Layer

[0054] The following components were used: Quantity Component [wt %] SBSrubber 68 plasticizer 23 monomer (hexanediol 7 diacrylate)photoinitiator, dyes 2

[0055] The components were dissolved in toluene, poured onto a PET filmprovided with a release layer and dried at 75° C. The residual solventcontent of the layer was 2%.

[0056] (c) Laminating

[0057] The dried photosensitive layer was laminated to the aluminumsheet provided with the adhesive layer. Just before lamination, thedried photosensitive layer was heated to a temperature of ca 60° C. bymeans of hot air; a laminating solvent was not used.

[0058] (d) Further Processing

[0059] Making the printing forme:

[0060] Following one week's storage the plates were processed toprinting formes. The following processing steps were carried out:

[0061] a. removal of the PET film,

[0062] b. positioning of a negative,

[0063] c. main exposure under a vacuum bag (F III imager, BASF, 3-24 minat 3 minute intervals),

[0064] d. plate washout (flowline system Combi LF II, BASF, flow rate150 mm/min, organic washout agent (based on hydrocarbon solvents)Nylosolv® II sold by BASF, 30° C., relief depth 1000 μm),

[0065] e. drying at 65° C. over a period of 2 hours,

[0066] f. re-exposure (UV-A, F III imager, BASF, 15 min),

[0067] g. posttreatment with UV-C radiation (F III detacker, BASF, 15min).

[0068] The steps a. to g. correspond to conventional processing stepsfor flexographic printing plates on transparent PET supports except thatthe back exposure step was omitted. The test printing forme wasevaluated. The plates were examined to determine the exposure time atwhich the positive test elements [freestanding dot 100 μm, screen 100 μmand the 3% halftone patch (24 L/cm)] were correctly formed on theprinting plate. The exposure time required to assure correct formationof all of the positive elements is referred to as the lower exposurelimit (LEL), ie the minimum exposure time for correct imaging.

[0069] Furthermore, the negative elements, ie the 500 μm negative dotand 2000 μm pit, were assessed. If the depth in the negative dot fallsbelow 70 μm or in the pit below 500 μm, correct ink transfer is nolonger guaranteed. The time at which at least one of the two negativeelements falls below these limits is referred to as the upper exposurelimit (UEL), ie maximum exposure time for an usable printing plate. Thedifference between UEL and LEL is the exposure latitude (EL), which mustbe positive for practical purposes.

[0070] The exposure latitude measured was +10 min and was thus positive.

[0071] The adhesion of the printing plates on the support was determinedas follows: to determine the adhesion of the support layer a plate isfirst of all cut to give a piece measuring ca 10×20 cm, which is thenfully exposed for at least 20 minutes over its entire area without theuse of a vacuum bag. 2 samples measuring 20 cm×2 cm (±0.1 cm) are thencut off using gate shears. These strips must be cut exactly, asotherwise false measurement results may arise. Before the test stripscan be clamped into the tension tester (Zwick) the relief layer of thesamples must be manually peeled away from the support over a distance ofca 5 cm. The tension tester peels the relief layer from the support atan angle of 180° and measures the force necessary for this purpose inN/2 cm. The results are summarized in Table 1.

EXAMPLE 2

[0072] Example 1 was repeated except that a support of sheet steel(thickness 0.14 mm) was used. There was obtained an adhesive layerhaving a thickness of from 12 to 15 μm. Examination under the microscoperevealed rubber particles of from 80 to 100 μm in diameter in acontinuous matrix. The adhesive layer on the support was tack-free. Thecoated steel sheets could be stacked without sticking together. Thedegree of adherence was lower than that achieved on sheet aluminum, butwas still sufficiently high to assure adequate adhesion in applicationsnormally encountered in practice. The results are summarized in Table 1.

EXAMPLE 3

[0073] Example 2 was repeated except that an adhesive coatingcomposition of polyvinyl alcohol, modified polyvinyl alcohol(ethylenically polymerizable side-groups), glyoxal andbis(N-methylolacrylamide)ethylene glycol ether was used. The adhesivecoating composition was crosslinked by heating to 180° C. over a periodof 2 minutes. The adhesive layer on the support was tack-free. Thecoated sheet steels could be stacked without sticking together. Thedegree of adherence on the sheet steel was excellent, the photosensitivelayer could not be separated from the support. The results aresummarized in Table 1.

COMPARATIVE EXAMPLE 4

[0074] Use was made of an adhesive coating composition intended forwater-developable high-pressure plates and comprising a polyamide and aphenolic resin, such as is described in DE-A 2,202,357. The adhesivelayer on the support was tack-free. The coated steel sheets could bestacked without sticking together but the adherence was only 2.7 N/2 cmand thus inadequate for practical requirements. The results aresummarized in Table 1.

COMPARATIVE EXAMPLE 5

[0075] Example 1 was repeated except that a conventional layer ofadhesive polyurethane was used. In addition, prior to imaging throughthe photographic negative (steps b. and c. of the general instructions)a back exposure was carried out with actinic light passed through thePET support. The adherence of the relief layer on the support was good.The exposure properties were like those of the plates on metallicsupports. The fact that no back exposure can take place in the processof the invention has no impairing effect on the properties.

[0076] The results are summarized in Table 1.

COMPARATIVE EXAMPLE 6

[0077] In a manner similar to that described in Example 1 there wasprepared a mixture of an SBS block copolymer (Kraton D-KX222, 19.5 wt%), an oligomeric polyurethane resin containing hydroxyl groups (NPP262, 35 wt %), an aromatic isocyanate (Desmodur L 67, 9.5 wt %) and anultraviolet absorber (hydroxy Michler's ketone, 1 wt %). The copolyester(Vitel 3300, 35 wt %) was replaced by a special adhesive resin (Regalit5100, Hercules). The mixture was dissolved in THF and poured onto analuminum support (thickness 0.3 mm), the solvent evaporated and theresidue crosslinked by heating at 170° C. for 2.5 min. The adhesivelayer was extremely sticky and the metal sheets could not be separatedfrom each other in a stacking test. The results are summarized inTable 1. TABLE 1 Results of Experiments and Comparative Examples.Example Example Example Comp. Ex. Comp. Ex. Comp. Ex. 1 2 3 4 5 6Support Aluminum Steel Steel Steel PET film Aluminum Adhesive SBS/PUSBS/PU PVA PA PU SBS/PU coating composi- tion Tack tack- tack- tack-tack-free tack- tacky free free free free Plate ad- >20 5.5 >20 2.7 >204.7 herence [N/2 cm] Shore Hard- 75 73 75 75 75 74 ness [° A] LEL [min]6 6 6 — 6 — UEL [min] 16 16 16 — 16 — EL [min] 10 10 10 — 10 — Remarksinade- with too quate ad- back ex- tacky hesion posure

1. A process for the manufacture of organically developable,photopolymerizable flexographic elements on flexible metallic supportsfor making flexographic printing plates, wherein the following steps arecarried out: (d) coating a flexible metallic support with a tack-freeadhesive layer which is insoluble and non-swelling in printing inks andorganic developers, (e) coating a protective film or a protectiveelement with an organically developable, elastomeric, photopolymerizablelayer by dissolution of at least one thermoplastic elastomeric binder,ethylenically unsaturated monomers, photoinitiator or photoinitiatorsystem and, optionally, other components in a suitable solvent, pouringthe solution onto the protective film or the protective element and thenevaporating the solvent, (f) laminating the elastomeric,photopolymerizable layer on its side remote from the protective filmwith the flexible metallic support coated with an adhesive layer.
 2. Aprocess for the manufacture of organically developable,photopolymerizable flexographic elements on flexible metallic supportsfor making flexographic printing plates, wherein the following steps arecarried out: (e) coating a flexible metallic support with a tack-freeadhesive layer which is insoluble and non-swelling in printing inks andorganic developers, (f) melt extruding an organically developable,elastomeric photopolymerizable composition at least comprising athermoplastic elastomeric binder, ethylenically unsaturated monomers,photoinitiator or photoinitiator system and, optionally, othercomponents, discharging said composition through a slot die between aprotective film or a protective element and a temporary supporting film,and then calendering the composite, (g) peeling off the temporarysupporting film, (h) laminating the photopolymerizable layer on its sideremote from the protective film with the flexible metallic supportcoated with the adhesive layer.
 3. A process as claimed in claim 1 or 2,wherein the flexible metallic support is a support comprising aluminum,steel or magnetizable spring steel.
 4. A process as claimed in claim 1or 2, wherein the photopolymerizable layer has a thickness of from 0.5to 1.5 mm.
 5. A process as claimed in any of claims 1 to 4, wherein theprotective element comprises a protective film and an IR-ablative layer.6. A process as claimed in any of claims 1 to 5, wherein the tack-freeadhesive layer comprises a migration-stable UV absorber.
 7. Anorganically developable, photopolymerizable flexographic elementcomprising a flexible metallic support and an organically developablephotopolymerizable layer which are bonded to one another by an adhesivelayer, wherein the adhesive layer is tack-free and insoluble andnon-swelling in printing inks and organic developers, the adhesive layercomprising an elastomeric binder which is embedded in a polymericmatrix.
 8. A flexographic element as claimed in claim 7, wherein thetack-free adhesive layer comprises a migration-stable UV absorber.